1. Field of the Invention
The present invention relates generally to the art of glass melting and, more particularly, to an improved apparatus and method for regulating the atmospheric pressure within the headspace above the molten glass in the working end of a glass melting furnace.
2. Description of the Prior Art
In a conventional glass melting furnace raw batch material and scrap glass or cullet are charged into one end of the furnace and molten glass is removed from its other end. The glass, in moving through the furnace, passes through a melting end and a working end which are contiguous with one another. Heat is applied over the upper surface of the bath of glass in the melting end for reducing the newly added materials to a molten state and integrating them into the flowing molten mass, and the molten glass is refined and cooled to a point where it can be removed from the furnace in a continuous ribbon in the working end.
It is common practice to separate the headspace above the molten glass in the melting end from the headspace above the molten glass in the working end by a partition wall or arch, so that the melting operation can be performed to best suit conditions therein without in any way affecting the molten glass in the working end. Accordingly, the headspace at the working end comprises a substantially closed chamber wherein pressure tends to build up because of processes occurring in the molten glass. Thus, this pressure build-up is not relieved by the exhaust system employed in the melting section of the furnace.
Practice has shown that as the pressure in the working end builds up to excessive levels, it tends to force furnace atmospheric gases into the area wherein the molten glass, by whatever process, is being formed into a final usable ribbon. This flow of furnace atmosphere tends, among other things, to produce optical defects in the glass ribbon known in the art as "wave" and "ream". Wave may be characterized as an undulating streak occurring repetitively on the surface of the glass while ream may be characterized as a narrow band within the glass that has an effective refractive index different from the surrounding glass. These optical defects may have their origin in the ongoing conditioning of the molten glass in the working end of the furnace. More particularly, wave and ream may occur when cold air drafts enter the working section and cool the top surface of the molten glass in a non-uniform manner. The non-uniform cooling of the top surface of the molten glass produces cold streaks and evidences itself as wave in the glass. Further, the bath of molten glass may be of slightly different composition and the non-uniform cooling of these compositions evidence themselves as ream in the final glass ribbon.
In the practice of this invention the pressurized atmospheric air in the working section of the furnace is maintained at a level which minimizes the above-mentioned causes of wave and ream in the finished glass ribbon. This is accomplished by maintaining the working end atmosphere at a pressure which is always slightly above the external atmospheric pressure, with no momentary reversals of this relationship as may occur in conventional furnaces. There is thus no ingress of cold external atmosphere and yet the internal atmosphere does not build up sufficient pressure so as to enter the sheet forming area.
The general concept of controlling an air pressure in a closed chamber by sensing the air pressure and then regulating the same by controlling the exhaust thereof is, of course, old. Also, it is known that the atmospheric air pressure in the melting area of a glass furnace can be controlled by dampering its chimney draft (see U.S. Pat. No. 3,373,007 and 3,584,851). However, the pressure requirement in the melting end of the furnace for efficient burner operation is different than the pressure requirement in the working end of a glass melting furnace which should be minimal for minimizing the occurrence of optical defects in a finished glass ribbon.